The present invention relates to pickup cartridges of the moving magnet type.
As is well known, in the 45-45 system records, the recording groove consists of two side walls disposed at 45.degree. to the record plane, in one of the side walls being recorded left channel signals and in the other being recorded right channel signals. Many approaches have been proposed to derive signals of both channels of records of this system. Among them there is an approach in which when a stylus of a pickup cartridge traces the record groove, a signal corresponding to the horizontal component of the stylus vibratory motion and that corresponding to the vertical component thereof are detected separately, and then processed to obtain right and left channel signals. More specifically, in signals of the 45-45 system record, there are the following relationships: EQU EH=EL+ER (1) EQU EV=EL-ER (2)
where EL is a left channel signal, ER is a right channel signal, EH is a signal corresponding to the horizontal vibratory component, and EV is a signal corresponding to the vertical vibratory component. The sum Ea of and the difference Eb between the signals EH and EV provide EQU Ea=EH+EV (3) EQU Eb=EH-EV (4)
From the equations (1) to (4), EQU Ea=2EL (5) EQU Eb=2ER (6)
Thus the right and left channel signals are obtained based on the signals EH and EV.
This method has been applied to pickup cartridges of the moving magnet type, and such pickup cartridges have already been put into commercial use. In this kind of moving magnet type pickup cartridge, a cantilever arm or stylus arm carring a stylus at its free end has a permanent magnet attached at its fixed end, and around the magnet there are disposed yokes, having coils wound around them, for separately detecting a horizontal component of the magnet vibratory motion and a vertical component thereof. More specifically, the yokes for detecting the horizontal vibratory component are disposed on the lateral sides of the magnet and the yokes for detecting the vertical vibratory component are disposed above and below the magnet whereby the vertical and horizontal components are efficiently detected. With such construction the pickup cartridge has an overall shape having the lower portion thereof projected downwards since one of yokes for detecting the vertical component of the magnet vibratory motion is disposed below the magnet, and hence it is necessary to increase the length of the stylus arm to prevent the casing of the pickup cartridge from coming into contact with the record disc. However, increase in the length of the stylus arm provides disadvantages in that the equivalent mass of the vibratory system increases, which results in the narrowing of its reproduction band in high frequency range, and in that the rotation angle of the stylus arm is reduced with the result that output voltages of the coils drop.
For eliminating such disadvantages of the pickup cartridge of the moving magnet type adopting the above-described electrical signal producing system, there has been proposed a pickup cartridge, shown in FIGS. 1 and 2, having its casing not projected downwards. (see U.S. Pat. No. 4,177,360 to Fujimoto et al.) In this pickup cartridge, stylus arm 2 carrying a stylus 1 at its one end is attached at the other end to a permanent magnet 3 in a cantilever fasion, the magnet being magnetized in the axial direction of the stylus arm 2 and secured to a holder of the cartridge whereby a vibratory system is formed. In a plane in which magnet 3 lies, there is disposed a yoke 4 having three legs 4a, 4b and 4c so that legs 4a and 4b are positioned to face the lateral sides of magnet 3, and leg 4c is positioned above magnet 3, the legs 4a to 4c being surrounded by coils 5a to 5d.
FIG. 3 illustrates another typical example of the well-known pickup cartridge similar to the above-described pickup cartridge in FIGS. 1 and 2 except in that its yoke 6 has three legs 6a, 6b, and 6c of a short channel shape, of which webs extend horizontally to the lateral sides and upper side of magnet 3, and are surrounded by coils 5a to 5d.
In such pickup cartridges, when magnet 3 is angularly moved about a central point thereof as a fulcrum which is on the center axis of magnet 3, an electrical signal corresponding to the horizontal component of the magnet motion is induced in coils 5a and 5b, and an electrical signal corresponding to the vertical component thereof is induced in coils 5c and 5d due to the relationship in position between magnet 3 and legs 4a to 4c (6a to 6c). Therefore right and left channel signals can be derived by connecting coils 5a and 5c so that a sum of the signals induced therein may be produced, and by connecting coils 5b and 5d so that a difference between the signals induced therein may be produced. In these pickup cartridges the casing thereof does not project downwardly since any leg of the yoke is not provided below magnet 3, and thus stylus arm 2 can be shortened.
In contrast with the above advantages, this type of pickup cartridge has the following disadvantage due to the above-described structure having no lower pole piece. Although when magnet 3 vibrates horizontally there is no problem, when it vibrates vertically, legs 4a and 4b (6a and 6b) of yoke 4 for detecting the horizontal signal are used as parts of a magnetic circuit as shown in FIGS. 4 and 5 and hence a magnetic flux corresponding to the vertical vibratory motion of magnet 3 leaks into those legs 4a and 4b. Thus, in the pickup cartridge of the type in which the sum of and the difference between the vertical signal and the horizontal signal are produced to derive right and left channel signals, it is inevitable to encounter a problem in that a cross talk is generated between the right and left channels